1. Field of the Invention
The field of art to which the claimed invention pertains is the catalytic cracking of hydrocarbons. More specifically, the claimed invention relates to a process for the passivation of contaminating metals on fluidized cracking catalyst.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in the reaction zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the reaction zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
A common problem in the FCC process is the gradual deterioration of the catalyst due to the deposition on the catalyst of metal contaminants, such as nickel, cobalt, iron and vanadium, contained in the hydrocarbon feed. These deposited metal contaminants increase the production of hydrogen, light gases and coke at the expense of the highly desirable distillate products.
The art teaches many techniques for dealing with these undesirable metal contaminants. Such techniques can be divided into two broad categories, one being the passivation of the contaminants by reacting the catalyst with a passivating agent, which converts the metal contaminants to a relatively innocuous form, and the other being the physical removal of the contaminants from the catalyst. Examples of techniques falling within the former category are as taught or claimed in U.S. Pat. Nos. 2,758,097 (reaction with phosphorus pentoxide); 3,711,422 (reaction with an antimony compound); and 4,025,458 (reaction with chemical complexes containing antimony). Examples of techniques falling within the latter category are as taught or claimed in U.S. Pat. Nos. 3,252,918; 3,324,044; 4,013,546; and 4,014,815.
In U.S. Pat. No. 2,575,258 there is claimed a method for treating an Fe.sub.2 O.sub.3 contaminated FCC catalyst by reacting the catalyst with a reducing gas which may comprise a hydrocarbon having not more than three carbon atoms per molecule to convert the Fe.sub.2 O.sub.3 to Fe.sub.3 O.sub.4. The text of the specification of this patent admonishes against conversion of any substantial amount of the Fe.sub.2 O.sub.3 to metallic iron since the latter "detrimentally affects the activity of the catalyst". The reaction conditions of this patent are carefully chosen, therefore, to cause the conversion of the Fe.sub.2 O.sub.3 only to the Fe.sub.3 O.sub.4 form.
U.S. Pat. No. 2,425,482 claims the treatment of regenerated FCC catalyst with light olefins for "improving the octane rating of the gasoline produced as a result of cracking with the so-treated catalyst." This reference makes no mention of contaminating metals, however, it does exemplify use of the invention in an FCC process to which is charged feedstock comprising a 500.degree. to 700.degree. F. gas oil cut of an East Texas crude. That particular gas oil is known to have a total nickel, cobalt and iron content less than 0.5 ppm by weight.
U.S. Pat. No. 4,176,083 discloses the purging of hot regenerated FCC catalyst with a fuel gas which may comprise C.sub.3 and lower boiling components of a hydrocarbon cracking operation. The purpose of the purging is to displace entrained CO.sub.2 containing flue gas from the catalyst prior to returning the catalyst to the reactor. This reference makes no mention of contaminating metals or even the particular charge stock employed.
There is also art that teaches reduction and sulfiding treatment of non-fluidized moving catalyst beds to temper the activity of certain desired metals, such as those contained in Group VIII of the Periodic Table, intentionally added to the catalyst. The processes in which these moving catalyst beds are used include processes requiring the presence of hydrogen, such as hydrocracking and reforming. U.S. Pat. No. 3,838,038 is an example of such art teaching reduction and sulfiding of a non-fluidized catalyst containing certain desired metals.
We have found an improved process for passivating an undesired metal contaminant on a fluid cracking catalyst for use in a fluidized cracking system.